This invention relates to electrocution proof devices such as ground fault circuit interrupter systems for use in protecting the operator of apparatus such as an appliance or tool.
More particularly, this invention relates to circuitry which is fail-safe and provides protection to the operator of a portable hand-held appliance or tool such as a hair dryer, styling wand, etc. from a predetermined current level introduced when a fault occurs.
Ground fault circuit interrupter systems are intended to sense small differences in current in normally balanced power lines or cables. These differences may be caused by a leakage of current from one of the line conductors to ground, thus depriving the return line of some of its normal current which would establish a balance zero difference in current at the sensor. As long as the difference current is below a predetermined level, typically about 0.005 ampere, power should normally be allowed to flow uninterrupted. If a larger difference current occurs, the circuit should be interrupted since it is then probable that a malfunction of insulation or perhaps a serious shock to a human being is occurring.
In the past, portable ground fault circuit breakers treated both the hot or line conductor and the neutral or ground conductor symmetrically, i.e. interrupted both conductors whenever a fault was present.
The interruption of current in both conductors was necessary in the case of a portable appliance since it was not known which of the conductors was electrically connected to the line or "hot" side of the receptacle.
Attempts to construct fail-safe circuit interrupter systems for portable appliances utilizing solid state technology and at a reasonable cost have also in the past encountered various difficulties. For example, solid state switching elements such as Triacs have the safety disadvantage that they tend to fail as a short thereby creating an unacceptable risk to the operator of the appliance. In any event, although solid state switches overcome many inherent disadvantages of mechanical switches, e.g. arcing, corrosion, size, etc., prior art approaches using such solid state technology still required solid state switching elements in both the line conductor side and the neutral conductor side of the system along with additional control circuitry. The need for fault protection on both the neutral and line conductor sides is due to the ground ambiguity problem. This adds to the cost and complexity of the system.
More particularly, in a normally wired house one of the conductors from the alternating current power source is generally grounded and referred to as a ground or neutral conductor and the other conductors are electrified with varying voltages with respect to it and called the line or hot conductors. Prior art approaches for protecting the operator of a portable appliance with an integral ground fault circuit interrupter required the interruption of both the line and the neutral conductors in the presence of a fault since, unless permanently wired, there was no practical way to tell which side of the A.C. receptacle was hot. Interrupting both conductors appeared to be the only practicle way to handle a portable appliance ground fault situation since it was not readily acertainable what was the polarity of the A.C. power source. Prior polarity sensors did not provide hot side interruption when improperly connected. Further, even if an operator knew conventional wiring procedures, such an educated guess would be dangerous since many older homes were wired without regard for polarity convention.
With the cost conscience manufacturing of appliances, tools, and the like it is desirable to avoid the cost and complexity of prior switching schemes which interrupted both conductors in the presence of a fault. Thus, it is desirable to develop a system which is fail-safe and comprises a single switching element which would be plugged into an A.C. receptable by an operator in a manner which necessitates the single switching element being connected in series with the line conductor. Other safeguards would be included to provide an operator with a fail-safe electrocution proof appliance.